It is essential to identify and quantitate hazardous physical, chemical or biological agents remotely before they achieve dangerous levels. The ideal sensing technology should be able to separately and selectively detect the largest subset of chemical and biological agents with high sensitivity. This has motivated the development of numerous sensing approaches. Many of these approaches rely on sophisticated analytical techniques such as mass spectrometry, fluorescence, Raman spectroscopy, etc., that have disadvantages of complexity and cost.
Improved approaches to chemical sensing preferably would use methods that do not require sample preparation and that allow the visual determination of the chemical species and their concentrations. In one example of such an approach, three-dimensional (3-D) polymerized colloidal crystal array (PCCA) hydrogel sensing materials used diffraction to measure analyte. These 3-D photonic crystal materials were fabricated through self assembly of face center cubic (FCC) arrays of colloidal particles followed by polymerization to embed the colloidal crystal array (CCA) within a hydrogel network. The PCCA was then functionalized with the appropriate molecular recognition agents. These sensing materials detect analytes such as creatinine, glucose, pH, organophosphorus compounds, amino acids and metal ions.
Diffraction is typically relatively weak for 2-D particle arrays, e.g. polystyrene particles in air back diffract 1-10% of the incident light, depending on the colloidal particle diameter. The diffraction efficiency generally increases with the magnitude of the 2-D particle arrays dielectric constant modulation. Very large diffraction efficiencies can be achieved from metallic 2-D particle arrays or if multiple dielectric 2-D particle layers are stacked to form a 3-D particle arrays.